Healthy temperate reefs worldwide are dominated by highly productive kelp forests, which provide complex habitat for diverse fish and invertebrates (Steneck and Johnson 2013). These systems in turn provide significant economic (fishing, tourism etc) and intrinsic (recreation, enjoyment etc) benefits to the communities of people who live adjacent to them (Bennett et al. 2016). The basis to these ecosystem goods and services is argued to be based on the productivity of kelp, for its habitat-forming qualities and for the productivity it contributes to marine food webs in the form of direct herbivory and indirect herbivory from the production of detritus (Krumhansl and Scheibling 2012, Bennett et al. 2016). Rates of macroalgae production on temperate reefs have been documented to be among the highest of any marine, terrestrial or even agricultural system globally (Mann 1973, Cebrian 1999), with a global mean production of 860 g C m-2 yr-1 and on average 82% of that production entering the food web as detritus (Krumhansl and Scheibling 2012). However, temperate reef ecosystems are undergoing rapid changes, including increased thermal stress from rising ocean temperatures (Wernberg et al. 2013) and poleward range shifts of herbivores (Vergés et al. 2014). These ecosystems have experienced widespread losses of habitat-forming macroalgae and kelp, in part resulting from these changes.
Fish play an important role in the ecology of these temperate rocky reefs and contribute great economic benefits to coastal communities (Robertson 1982, Jones and Andrew 1990). In South Africa for example, the market for recreational and commercial fishing from temperate reefs is estimated to be worth US$256 million year-1 (Blamey and Bolton 2018) and in Australia, temperate reef recreational fisheries alone are estimated to be worth US$392 million year-1 (Bennett et al. 2016). The fish which are most regularly targeted and extracted from temperate reefs, the large piscivores, tend to be most favoured by recreational and commercial fishers, however many of these fisheries have experienced major decline over the 20th century and fishing lower down the food web has become almost essential in many parts of the world (Pauly et al. 1998). Planktivorous fish, which form the primary natural prey for piscivores (Scharf et al. 2003), are becoming increasingly targeted as a result of this decline. Herbivorous fish on the other hand are generally less diverse and abundant in temperate regions and tend to be less favoured by fishers. It is unknown how changing ocean conditions will influence the relative biomass and interactions of these trophic groups. In order to address this gap, we must uncover more about the trophic role that reef-fish play in this environment.
As all food webs are ultimately fuelled by autotrophs at their base, the sources of production which support fish biomass at temperate reefs can be either planktivorous (phytoplankton and the zooplankton which consume them) or benthic (macroalgae, turfing/encrusting algae, seagrass and detritus). The relative proportions of biomass made up by consumers which are highly dependent on a specific resource, such as zooplankton or macroalgae, should be an indicator of the primary sources of energy in an environment. As an example, a recent localised food web study of temperate rocky reefs found that 41% of fish biomass was composed of planktivorous fish, and 53% of the total fish biomass was supported either directly or indirectly by consumption of zooplankton (Truong et al. 2017). By comparison, only ~16% of biomass was made up of herbivorous fish, Given the disparity in proportional biomass for these two groups, it is unlikely that benthic energy sources form the trophic basis for this region.
Hamner et al. (1988) demonstrated that planktivorous fish are able to effectively deplete the zooplankton concentration flowing over a coral reef crest through daytime feeding, an ecological process which they coined as ‘Wall of Mouths’. It could be assumed then, that such a system could only support such a large proportion of planktivores through the regular delivery of zooplankton and would also be dependent on sufficient current strength to ensure that zooplankton supplies are replenished as they are depleted but not so strong that it would inhibit the ability of fish to feed (Hobson and Chess 1978).
Clearly a broader spatial and temporal scale would be useful in determining if this dependence on zooplankton is consistent and widespread for temperate reef food webs. As the delivery of zooplankton to coastal areas is dynamic (Pershing et al. 2005) and likely to be affected by climate change in the near future (O’Connor et al. 2009), the high dependence of this ecosystem to the delivery of this essential resource suggests that there is a need to monitor the variable nature of temperate reef systems, for all trophic groups, and assess their underlying spatial and temporal dynamics.
In Australia, temperate rocky reefs and kelp forests occur in a relatively continuous band which fringes the entire coast south of ~28° S to the southern tip of Tasmania across 16 degrees of latitude (Bennett et al. 2016). This marine ecosystem sits adjacent to approximately 70% of Australia’s population and it has been estimated that it contributes at least US$7 billion per annum to coastal communities from fishing and tourism alone (Bennett et al. 2016). The southeast coast of Australia, more specifically, is a climate change hotspot and has been used extensively as a model system to examine latitudinal patterns in coastal ecosystems due to its accessibility, its relatively north-south orientation and its prominent western boundary current, the East Australian Current (Malcolm et al. 2007, Poloczanska et al. 2011, Wernberg et al. 2011, Sunday et al. 2015, Bracewell et al. 2018). This region presents an ideal system to study patterns for space-for-time substitution (Blois et al. 2013), with the implication that present-day patterns are expected to shift poleward and in some cases this has already been documented (Johnson et al. 2011, Last et al. 2011).
The goal of this study was to describe large-scale latitudinal and seasonal patterns in the trophic composition of temperate rocky reef fish communities. We used the east coast of Australia as a model system and built upon the work of Truong et al. (2017), which highlighted the dominance of planktivory as the main trophic pathway for shallow reef food webs in the Sydney region. This study was enabled through use of the Reef Life Survey dataset (RLS). Specifically, the aims were to (1) determine if regions can be characterised by differences in the fish trophic composition along a latitudinal extent and compare this to latitudinal differences in species composition; (2) describe latitudinal-seasonal patterns in the trophic composition of temperate reef fish biomass; (3) determine how reef fish trophic composition and biomass are affected by ecological and environmental variables.